Corrosion is a known problem in a number of industries. In the oil and gas industry (O&G) alone, corrosion costs US refineries over $4 billion annually. Periodically depositing a corrosion resistant surface onto existing equipment is generally an economical method for protecting metallic components in aggressive environments, e.g., corrosive environments containing strong acids such as sulfuric acid, or bases at elevated temperatures. The coating is typically deposited using a thermal spray process. The technique is commonly used to protect refinery vessels, power generation equipment, chemical processing baths, and other large scale industrial surfaces.
In coatings made by a thermal spray process such as twin wire arc spray (TWAS), elemental components particularly the powdered species of the cored wire can oxidize (“in-flight particle oxidation”). Oxidation of the atomized molten thermal spray material is undesirable for several reasons, including: a) selective oxidation of alloying elements such as chromium, which reduces the corrosion performance of the deposited coating; b) the oxides embedded within the coating are not effective at sealing porosity in service; and c) high oxide content generally decreases both the adhesion of the coating to the substrate and the inter-particle adhesion. TWAS coatings generally contain a high degree of porosity in the range of 5%40%, and oxide content in the range of 5-10%. Such a high level of porosity inevitably leads to what is termed “through-porosity” or “inter-connected porosity,” meaning the coating is permeable to corrosive media leading to corrosion attacks regardless of the inherent corrosion performance of the thermal spray coating alloy. Additionally, corrosive media trapped in small pores can result in aggressive localized attack. As such, it is desirable to reduce the oxide content in thermal spray coatings.
There are a number of references disclosing thermal spray coating compositions. U.S. Pat. No. 4,561,892 discloses the use of a powder alloy of specific composition used in the plasma thermal spray process to deposit a corrosion resistant coating. U.S. Pat. No. 5,120,614 discloses a Ni—Cr-refractory type alloy to resist high temperature oxidation and acid attack suitable for use as bulk or weld overlay materials. U.S. Pat. No. 4,027,367 discloses nickel-aluminum alloy compositions for arc spray applications, forming a self-bonding coating. U.S. Pat. Nos. 4,453,976; 4,529,616, and 5,326,645 disclose powder alloys for use in thermal spray and flame spray applications. U.S. Pat. Nos. 2,875,042 and 7,157,151 disclose compositions for use in spray and fuse technique to form coatings.
There is still a need for coatings with improved characteristics in as-sprayed condition. There is also a need for improved methods to apply coatings, particularly for coating large surface areas on-site. The invention relates to improved compositions for thermal spray techniques, providing coatings with low porosity/oxide content.